1. Field of Invention
The present invention relates to a wireless network communications method, and more particularly, to virtual carrier sensing in a communications network using various data transmission rates, and a wireless communications method using the same.
2. Description of the Related Art
Recently, there is an increasing demand for ultra high-speed communication networks due to widespread public use of the Internet and a rapid increase of multimedia data. Since local area networks (LAN) emerged in the late 1980s, the data transmission rate over the Internet has drastically increased from about 1 Mbps to about 100 Mbps today. Thus, high-speed Ethernet transmission has gained popularity and wide spread use nowadays. Up to now, intensive research in the area of gigabit-speed Ethernet has been ongoing. An increasing interest in wireless network connections and communications has triggered research and implementation of wireless local area networks (WLAN). Now, there is an increasing availability of the WLAN to consumers. Although use of WLAN may be obstructed due to performance deterioration in terms of lower transmission rates and poorer stability compared to wired LAN, WLAN has various advantages, including wireless networking capability, greater mobility and so on. Accordingly, markets of the WLAN have been growing.
Due to the need for a greater transmission rates and the development of wireless transmission technology, the initial IEEE 802.11 standard, which specifies a 1-2 Mbps transfer rate, has evolved into more advanced standards including 802.11b, 802.11g and 802.11a. Recently, conferences for establishing the new IEEE standard, 802.11g have been held. The IEEE 802.11a standard, which specifies a 6-54 Mbps transmission rate in the 5 GHz-National Information Infrastructure (NII) band, uses orthogonal frequency division multiplexing (OFDM) as transmission technology. With an increasing public interest in OFDM transmission and use of 5 GHz band, much greater attention has been paid to the IEEE 802.11a than other wireless LAN standards.
Recently, wireless Internet services using WLAN, so-called “Nespot”, have been launched and offered by Korea Telecommunication (KT) Corporation, Korea. Nespot service provides access to the Internet using a WLAN according to IEEE 802.11b, commonly called Wi-Fi representing wireless fidelity. Communication standards for wireless data communication systems, which have been completed and promulgated or have been under research and discussion, include WCDMA (Wide Code Division Multiple Access), IEEE 802.11x, Bluetooth, IEEE 802.15.3, etc. known as 3G (3rd generation) communication standards. The most widely known, cheapest wireless data communication standard is IEEE 802.11b, a series of IEEE 802.11x. An IEEE 802.11b WLAN standard provides data transmission at a maximum rate of 11 Mbps and utilizes the 2.4 GHz-Industrial, Scientific, and Medical (ISM) band, which can be used under a predetermined electric field without permission. With the recent widespread use of the IEEE 802.11a as WLAN, which provides a maximum data rate of 54 Mbps in the 5 GHz band by using OFDM, IEEE 802.11g developed as an extension to the IEEE 802.11n for MIMO (multiple input multiple output) is being researched intensively.
The Ethernet and the WLAN, which are currently being widely used, both utilize a carrier sensing multiple access (CSMA) method. The CSMA method is used to determine whether a channel is in use or not. When it is determined that the channel is not in use, that is, when the channel is idle, then data is transmitted. If the channel is busy, retransmission of data is attempted after a predetermined period of time. A carrier sensing multiple access with collision detection (CSMA/CD) method is an improvement of the CSMA method, which is used in a wired LAN, whereas a carrier sensing multiple access with collision avoidance (CSMA/CA) method is used in packet-based wireless data communications. In the CSMA/CD method, a station suspends transmitting signals when a collision is detected during transmission. Unlike the CSMA method which pre-checks whether a channel is occupied or not before transmitting data, in the CSMA/CD method, the station suspends transmission of signals when a collision is detected during the transmission of signals and transmits a jam signal to another station to inform the occurrence of the collision. After the transmission of the jam signal, the station must wait for a random backoff period and then restarts transmitting signals. In the CSMA/CA method, the station does not transmit data immediately after the channel becomes idle but must wait for a random backoff period after a predetermined duration before transmission to avoid collision of signals. If a collision of signals occurs during transmission, the duration of the random backoff period is increased twofold for lowering the probability of collision (interference).
FIGS. 1A and 1B illustrate a conventional process of transmitting and receiving a frame in a contention period. A frame is received in a station under the assumption that the received frame has been transmitted to another station as a receiving station.
First, referring to FIG. 1A, a frame transmitted through a channel is received by a station without error. A station cannot transmit a frame through a channel while the frame is being received in another station: this method is referred to as physical carrier sensing. A medium access control (MAC) header of the received frame contains duration information. This duration information contains a duration of time taken from transmission of a frame by a transmitting station to reception of an acknowledge (ACK) frame from a receiving station. The receiving station receives the frame transmitted from the transmitting station and transmits the ACK frame after a short duration, known as a short inter-frame space (SIFS), to the transmitting station. A station sets a network allocation vector (NAV) using duration information, This method is called virtual carrier sensing. In order for a station to transmit a frame to another station, the station waits for a distributed inter-frame space (DIFS) after the lapse of an NAV period of time, and then performs a random backoff, and finally transmits the frame. When a carrier is sensed in a medium while performing the random backoff, however, the station suspends the random backoff, and waits until the channel is empty. Then, the station waits for a DIFS, and performs the random backoff.
Referring to FIG. 1B, a station cannot receive a frame transmitted through a channel. A station cannot utilize a channel while a frame is being transmitted through the channel, which is called physical carrier sensing. When a frame transmitted through a channel cannot be received due to the occurrence of an error, the station cannot set an NAV value because the NAV value is provided as information loaded in the frame. Thus, before transmission of a frame, the station that is unable to set an NAV value must wait for the duration of an extended inter-frame space (EIFS), which is longer than a DIFS, and then perform a random backoff. In FIG. 1B, when the channel becomes idle due to a failure in receiving a frame, the station waits for an EIFS. An ACK frame corresponding to the frame is transmitted through the channel before the EIFS, that is, immediately after an SIFS. If the station cannot receive even the ACK frame, the station must wait for another EIFS which starts after duration of ACK frame and finally performs a random backoff when a channel is idle, so as to transmit the frame. In other words, when the station cannot perform a virtual carrier sensing because of failure in obtaining a NAV value, the station has to wait longer than when the frame is received with no error. Because of this, the probability of the station losing in the contention of frame transmission would increase, thereby adversely affecting data transmission efficiency. Occurrence of such an error may be more distinguishable in a communication environment in which various modulation schemes and coding rates are used, like in IEEE 802.11a communications. In other words, when frames are transmitted to a station at a rate which is not supported by the station, the station cannot interpret the transmitted frame so that duration information cannot be obtained from an MAC header of the frame. Consequently, virtual carrier sensing from a frame transmitted at a rate not available for reception by a station cannot be achieved, resulting in deterioration of station performance.
In more detail, problems associated with conventional virtual carrier sensing will be described with reference to FIGS. 1A and 1B.
Unlike physical carrier sensing, virtual carrier sensing (VCS) assumes that a medium is occupied for a predetermined duration. Whereas physical carrier sensing is achieved based on measurement of actual wireless media, virtual carrier sensing is performed such that a predetermined value selected among received/transmitted data is set, duration of a medium occupation is estimated using the selected predetermined value, and transmission of data is then started after the estimated duration has elapsed. That is, unlike physical carrier sensing, virtual carrier sensing cannot be properly performed when data is not successfully received. In a normal virtual carrier sensing operation, as shown in FIG. 1A, when a network allocation vector (NAV), as information necessary for virtual carrier sensing, is received normally, it is possible to identify how long the medium will be occupied (busy), by reading the NAV value. On the other hand, when an error occurs, that is, when there is an error in reading a received frame, as shown in FIG. 1B, a NAV value cannot be read from the frame. Thus, the station has to wait for longer than a NAV period, for instance, an EIFS (Extended Inter-Frame Space) according to IEEE 801.11a.
Now, the reasons why the problems stated above have been generated will be described with reference to FIG. 2.
FIG. 2 illustrates conventional carrier sensing, supporting two kinds of carrier sensing structures: physical carrier sensing and virtual carrier sensing. As to a physical sensing structure, information stored in a physical layer 210 has a structure 212. A physical layer convergence procedure (PLCP) preamble 214 is a PLCP synchronization signal for the purpose of informing in advance which data in the physical layer 210 will be transmitted. A signal 216, as indicated by SIGNAL, is preceded by the PLCP preamble 214, and SIGNAL is modulated by a basic modulation scheme, and carries information which is necessary to receive a next data field 218, as indicated by DATA. The SIGNAL 216 will later be described in detail with reference to FIGS. 5A-C. Information contained in the SIGNAL 216 has a segment, as indicated by RATE, corresponding to a modulation scheme used in transmission of the DATA 218. This information enables data transmission/reception using various modulation schemes. As shown in FIG. 2, physical carrier sensing is implemented based on whether the medium receives a certain signal or not. Upon receiving the PLCP preamble 214, the physical layer 210 informs a medium access control (MAC) layer 220 that the physical layer 210 is busy, through a busy signal 222. Also, at an instant when the reception of data is terminated, that is, when the channel becomes idle as indicated by reference numeral 228, the physical layer 210 informs the MAC layer 220 that the use of the physical layer 210 is terminated. In the physical carrier sensing, however, transmitting data to an arbitrary station may not be accepted by another station. In this case, it is necessary to perform virtual carrier sensing. In virtual carrier sensing, a duration (NAV) value in an MPDU of the DATA 218 is read by the MAC layer 220 to recognize whether the medium is busy for a corresponding duration. Here, the MPDU, which is an abbreviation for MAC Protocol Data Unit, refers to data adopted by an MAC to transmit it to another MAC connected to the network. However, the NAV value can be read only when a data field is received normally. Therefore, if a receiving station is only able to receive a signal field but is not able to read the data field modulated using various schemes, a NAV value set in the data field cannot be read.
What is needed is a communication method capable of improving the performance of a station by ensuring virtual channel sensing using frames transmitted at various rates.